The reactive monomer-dispersed silica sol of the present invention is a silica sol in which colloidal silica particles are homogeneously dispersed in a polymerizable organic compound monomer cured by the irradiation of activating energy rays such as ultraviolet (UV) rays, electron beams, γ-rays and X rays, heating, the use of catalysts or the use of curing agents, or the like. Here, curing is referred to as the phenomenon of polymerization or crosslinkage.
A reactive monomer-dispersed silica sol is ultimately converted into a composite material of colloidal silica particles and a polymer by polymerizing the silica sol to which curing catalysts and other additives are added with UV rays or heat, and the composite material is used in applications such as a hard coating for a transparent plastic plate, a lens, a bottle, a film etc., various sealants and synthetic resin-molded articles. At this time, the colloidal silica particles act as a modifier for the polymer and improve, for example the elasticity modulus, the linear expansion coefficient, the glass transition temperature, the hardness, the flexural strength etc. of the polymer. In addition, when the colloidal silica particles have small particle diameters and are advantageously dispersed in a polymer matrix, the above modifying effects can be obtained without impairing the transparency of the polymer. Typically, it is known that when colloidal silica particles having a primary particle diameter of  50 nm or less are homogeneously dispersed in a polymer matrix, a composite material having high transparency in which the optical loss is extremely small can be obtained.
As a method for obtaining a reactive monomer-dispersed silica sol, there is known a method including mixing an organic solvent-dispersed silica sol etc. with a reactive monomer and removing at least a part of a solvent such as the organic solvent (refer to Patent Document 1).
When a reactive monomer-dispersed silica sol is prepared by the method in Patent Document 1, by an action of the solid acidity which the surfaces of the colloidal silica particles have, a polymerization or degradation of the reactive monomer may be caused and the stability of the sol may be impaired. In addition, in a cured article of a curable composition using a reactive monomer-dispersed silica sol, the deterioration, degradation, or the like of the polymer may be caused and a discoloration, a crack, or the like may be generated by an action of the solid acidity of colloidal silica particles.
[Patent Document 1]
Examined Japanese Patent Application Publication No. JP-B-7-35407